EFFECTS OF X-RAYS AND RADIOMIMETIG 



AGENTS ON NUCLEIC ACIDS AND 



NUCLEOPROTEINS 



J. A. V. Butler 



Chester Beatty Research Institute, Institute of Cancer Research, 

 Royal Cancer Hospital, London 



I INTERPRET my fuHction as being to give an account of 

 present views on nucleic acid and nucleoprotein structure and 

 the chemical effects of irradiation, as a background for the 

 more specialized discussions which will follow on the actual 

 effect of ionizing radiations on metabolic processes in which 

 these substances are known or suspected to take part. The 

 discussion of DNA must begin with the structure proposed by 

 Crick and Watson (1953) which, although it may be subject 

 to modification in some minor respects, has proved adequate 

 so far to accommodate the known facts. In this structure, as 

 is well known, two complementary nucleotide chains are held 

 together by hydrogen bonds between the bases guanine and 

 cytosine and adenine and thymine. Numerous measurements 

 of molecular weight have given values of the order 6-8 X 

 10^. This implies a chain length of approximately 5 X 10* A 

 or 5 X 10"* cm. As determined by physical measurements, 

 the actual length of the particle is considerably less than this, 

 viz. 4-6 X 10^ A (Sadron, 1955). It follows that the particle, 

 although a fairly rigid structure, must be bent or coiled. 

 When studied by Shooter and Butler (1955) in the ultra- 

 centrifuge at low concentrations, a -very considerable range of 

 sedimentation constants was observed (often from S = 10 to 

 S = 40), so that there must be present a variety of fixed 

 shapes or sizes. Since one source (calf thymus) has given a 

 variety of products, we have to conclude that the product is 

 sensitive to the mode of preparation {e.g. by enzyme actions). 



It must also be realized that the structure of DNA is not a 



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